The invention relates to an extruder/gear pump assembly having an extruder screw and a gear pump coupled thereto.
Constructions consisting of gear pumps and extruders have been known for a long time. In the case of the gear pumps connected to the output side of extruders, a fairly high feeding pressure can be built up by means of the gear pump. A corresponding example is the solution known from U.S. Patent document U.S. Pat. No. 4,642,040. In this solution, the extruder screw can be connected to the external shaft, which is connected with a variable transmission. A gear pump can be driven by way of a special transmission. In this solution, a diagonal duct is provided on the output side of the extruder, which has the purpose of feeding the gear pump. This solution illustrated in FIG. 6 of U.S. Pat. No. 4,642,040 requires fairly high expenditures. For example, the duct between the extruder screw and the gear pump may require additional heating.
In another embodiment of this solution known from U.S. Pat. No. 4,642,040, the gear pump is driven by way of a special shaft arrangement, the drive connection branching off the driving motor for the extruder screw. Here, a multiple diversion by way of slave transmissions and, additionally, the use of a variable transmission are required. The patent document already implies that further development with various and, primarily, long external shafts in that document is disadvantageous.
Various attempts to improve this solution have become known. For example, another combination of an extruder and a gear pump has become known from European Patent document EP-A1-508 285. This construction is aimed particularly at the further development of a gear pump and provides a separate drive for the gear pump. In this case, it is a disadvantage that the two drives must be mutually synchronized in a special manner in order to achieve the desired pressure conditions.
Furthermore, a combination of an extruder and a gear pump was suggested in German Patent document DE-A1 100 49 730, in which the gear pump is flangedly connected directly to the extruder screw. This solution avoids long flow paths, is compact and, therefore, not susceptible to disturbances. However, it would be desirable to obtain a better sealing-off of the high-pressure area. Thus, on the output side of the gear pump, this solution additionally has the serious disadvantage that the rotational speed of the gear pump is coupled to the rotational speed of the extruder screw. However, for different rubber and thermoplastic mixtures, different relative rotational speeds are required, so that this solution can be used only for one rubber or thermoplastic mixture or for a rubber or thermoplastic mixture of the same type.
It is, therefore, an aspect of the invention to provide a combined extruder/gear pump assembly, having an extruder screw and a gear pump coupled thereto, which avoids the disadvantages of long flow paths, in which case a controlling of the operating point should nevertheless be possible without requiring a plurality of drives.
According to the invention, a gear pump shaft is disposed in the hollow extruder screw, by way of which gear pump shaft the relative speed of the extruder screw and the gear pump can be adjusted. Advantageous further developments are described and claimed herein.
Extruders frequently have a considerable length, particularly when they are used for mixed purposes and, therefore, for example, have transfer areas. An extruder screw intended for this purpose will then no longer be self-supporting but run in the extruder housing. In order to improve the efficiency of the mixing, it is advantageous to select a relatively large diameter for the extruder screw at its outer circumference. This is accompanied by an enlargement of the diameter of the extruder screw core. In order to minimize the abrasion in the screw housing, hollow extruder screws are advantageous.
Such extruder screws have the advantage of a comparatively low weight and of a good rigidity, so that the wear between the extruder screws and the extruder housing is reduced.
An extruder screw which has a correspondingly favorable construction can very advantageously be used according to the invention.
The extruder screw constructed as a hollow shaft is used for the accommodation of a drive shaft for the gear pump. A large number of advantages can be achieved by means of this surprisingly simple measure.
First, a separate sealing-off of the hollow shaft at its end can be eliminated. By way of the gear pump, the extruder screw can be disposed on the end side, so that the wear by friction between the screw housing and the extruder screw is particularly low.
As a result of the implementation of a hollow shaft, the tendency to bend is considerably reduced even in the case of fairly long extruders. According to the invention, it is particularly favorable that the gear pump can be controlled despite the integration in the extruder screw. It is particularly advantageous that, as a result of the automatic taking-along of the planet carrier with the extruder screw, only the relative rotational speed and, thus, the relative delivery rate can be adjusted. Therefore, when the extruder and thus the main drive moves down, the planetary gear pump is also controlled down, so that the control is simplified.
For this purpose, it is particularly advantageous for a series planetary gear set to be provided between the gear pump drive shaft and an intermediate shaft, whose ring gear on the inside is formed by the extruder screw. Thus, different compounds and thermoplastic mixtures can also be optimally delivered without the risk of damage to the material.
Furthermore, it is particularly advantageous according to the invention that, despite the use of an internal transmission for providing the adjustability of the relative rotational speed, only a single sealing device is required for protecting the transmission from the elastomer masses. In addition, this sealing device is situated on the input side of the gear pump, thus not yet in the high pressure area, so that it is less stressed than a sealing device in the high-pressure area. In practice, it is additionally situated in the area of the suction side of the planetary gear pump, which further reduces the stressing of the sealing device.
By providing inlet and outlet openings adjacent to the outer circumference of the planetary gear pump, a favorable rate of delivery is achieved while the shearing stress to the material is, simultaneously, not excessively high, and the material exiting from the gear pump can be removed by way of an outlet cone in a manner advantageous to the flow.
Additional advantages, details and characteristics are contained in the following description of an embodiment of the invention by means of the drawing.